1. Field of the Invention
The present invention concerns a magnetic resonance (MR) head coil suitable for use in examinations of the head of a patient in magnetic resonance tomography. Head coils of the above type are in particular used in fMRI (functional magnetic resonance imaging) methods.
2. Description of the Prior Art
An MR head coil having a helmet-shaped coil housing with an arrangement of a number of antenna elements being mounted on the coil housing, and wherein the coil housing is shaped so as to completely free the field of view of a patient wearing the coil housing, is known from EP 0 148 566 A1.
For magnetic resonance examinations of the head of a patient in a magnetic resonance tomography, it is desirable to place antenna elements that serve for excitation and/or readout of magnetic resonance signals optimally densely on the head surface. An antenna element carrier in the form of a helmet closely abutting the head is frequently used for this purpose, on which are arranged antenna elements such that they ensure a complete head imaging. For this purpose it is required that antenna elements are also placed in the field of view of the patient.
FIG. 5 shows a helmet-shaped coil housing with antenna elements according to the prior art. In the following the combination made up of coil housing and antenna elements is called a head coil. The head coil 1 shown in FIG. 5a has a uniformly distributed arrangement of a number of first antenna elements 2 that are arranged on or in a coil housing 3. In order to be able to also generate and/or read out magnetic resonance signals in the field of view of the head of the patient 4, the helmet-shaped coil housing 3 is pulled down below the eyes of the patient in the field of view, with two coil elements arranged like eyeglasses. Gaps 5 through which the patient can look are provided inside the antenna elements 2 in the coil housing 3 in the field of view of the patient 4.
In a perspective, schematic view, FIG. 5b shows the helmet-shaped coil body placed on a patient head.
The gaps 5 significantly impede the view of the patient. In order to prevent or mitigate claustrophobic feelings of the patient, eyeglasses with a mirror tilted by 45° are often placed on the patient so that, while situated in the scanner tube, the patient can look out from the tube. The attachment of such eyeglasses is hindered or made entirely impossible with the head coils shown in FIGS. 5a and 5b. 
A free view is particularly important for the patient in functional imaging (fMRI). Among other things, films, images or a text for stimulation are shown to the patient, and any limitation of the field of view hinders the patient's intake of information and influences the result. The problem of the viewing obstruction is aggravated the more (and consequently smaller) antenna elements that are placed on the coil housing 3 around the head. This is particularly relevant since an emerging development is the use of head coils with low channel numbers, i.e. a lower number of antenna elements (for instance 1 to 4), up to head coils with 12 antenna elements and even up to 32-channel head coils, i.e. those with 32 antenna elements. Due to the larger number of the antenna elements the individual elements are inevitably smaller and it is thus nearly impossible to offer the patient a free field of view in the manner shown in FIGS. 5a and 5b. For 12-channel head coils it helps that the antenna elements in the viewing region of the patient were enlarged in diameter relative to the remaining antenna elements, such that the gaps 5 could be made correspondingly larger. The different size of the antenna elements, however, can lead to signal inhomogeneities in the head and performance degradation.
Due to the small antenna element size, the future multi-channel coils (for example head coils with 32 antenna elements and more) have in the viewing region a plurality of elements that can also overlap. These viewing elements are not necessary for functional imaging (in which, as already mentioned, an unoccluded view of the patient from the coil is necessary) and can therefore be removed.
EP 0 148 566 A1 describes an MR head coil in the form of a helmet that frees the field of view. Mounting of additional coils or control elements is not provided.
Similar head coils are also known from WO 2005/109010 A2 and DE 195 05 062 A1. Magnetic field inhomogeneities due to the missing coils in the field of view are accepted.
EP 0 437 049 A2 describes a coil arrangement for eye examination of patients. The MR coils are either a type of contact lens that is inserted into the eye or mounted on an eyeglasses frame. Here exclusively an eye examination is intended.